Abrasive composition of matter and method of forming same



Patented Jan. 16, 1945 PATENT GFFICE ABRASIVE COMPOSITION OF MATTER AND IVIETHOD F FORMING SAME No Drawing. Application July 9, 1943, Serial No. 494,077

4 Claims.

Another object is to provide an abrasive .com-

position of matter of the type described which is adapted to be soldered or weld secured upon the working face of a grinding or cutting tool.

Still another object is to provide an improved lens grinding tool.

Other objects will be apparent as the invention is more fully hereinafter described.

In accordance with the above objects I have discovered that by dispersing the diamonds throughout a metallic body consisting of a porous sintered metal framework impregnated with a silver solder alloy, the objectives of the present invention may be obtained.

Dispersal of the diamonds throughout the metallic body consisting of a porous sintered framework impregnated with a silver solder alloy is obtained by first surfacing the diamond particles with a metal that is readily alloyable with a molten silver solder alloy, such as Ag, Au and Cu, dispersing the metal surfaced diamonds throughout a mixture of metal powders consisting of a refractory metal and a bonding metal, compacting the diamond-metal powdenmixture, and heattreating the compacted product to a temperature at'which the bonding metal is effective in sintering the refractory metal into a strong porous metal body, and then immersing the sintered body into a molten silver solder alloy to fill up the rores of the porous body and to alloy secure the diamonds in their dispersed position therein.

The practice of this method islimited to a maximum temperature of heating approximating 800-850 C. depending upon the size of the diamond particles employed. This is the maximum temperature of heating that may be applied to such diamonds before thermal decomposition of the same occurs, particularly where the particle size of the diamond fragments is below 100 mesh, as is generally the case in abrasive compositions of matter of the type included within the scope of the present invention.

In order, therefore, to obtain a strong porous sintered metallic body for impregnating with the molten silver solder alloy, I have found it necessary to employ a metal such as iron or nickel as bonding agents for the refractory metals tungsten and molybdenum. In general, from 10 to 20% of the iron or nickel metal powders in admixture with tungsten or molybdenum metal powders appears adequate for the purpose of obtaining a sintering of the metal powders at temperatures' approximating 800-850 C. The particle size of the metal powders and the amount of pressure employed in compacting the same, also are factors entering into the matter, as one skilled in the art will perceive.

The metal powders utilizable in the present invention are those which have been produced by the hydrogen reduction of the metal oxides at relatively low temperatures, and in the case of the iron and nickel metal powders, those which are obtained by the thermal decomposition of the carbonyl compounds of these metals.

The particular method of surfacing the diamonds with the metal readily alloyable with silver solder and the thickness of the same appears immaterial. The primary function of the surfacing metal in addition to being readily alloyable with silver solder is to protect the diamond from reaction with the refractory and bonding metal powders during sintering and from oxidation. Any thickness of metal coating effective to produce this result is satisfactory for the purpose ofthe present invention.

The application of this metal coating onto the surface of the diamond can be accomplished by the practice of a plurality of different methods old and well known in the art and, per se, not forming a part of the present invention, such as by electro-deposition, vaporization, thermal dissociation and by electronic projection. The latter method is preferred and in the practice of this method the invention described and claimed in my Patent No. 2,103,623 dated December 28, 1937, is followed.

The particle size of the diamond fragments employed in the present invention may vary Widely without essential departure from the present invention, however. The particle size difference between the diamond fragments preferably is maintained relatively low. The best particle size range to employ for most abrasive and cutting tool compositions appears to be that passing mesh but not passing mesh.

The relative proportions of the tungsten or molybdenum metal powder and iron or nickel metal powder mixture and metal surfaced diamonds to be employed also may vary widely elepending upon the abrasive efiiciency and strength and rigidity desired in the sintered composition. Generally speaking a ratio of 1 part diamond to from 5 to parts of the metal powder mixture (by weight) has been found adequate for most purposes. Where lens grinding tools are being formed, however, the 1 to 10 ratio appears to be the most satisfactory from all viewpoints.

The amount of pressure to employ in compacting the metal surfaced diamond-metal powder mixture also may vary widely without essential departure from the invention as one skilled in the art will recognize. It is undesirable to employ a pressure that is high enough to cause the metal surface on the diamond to be perforated by the adjacent metal powders. On the other hand a sufficient pressure with any given metal powder grain size must be employed to obtain grain growth and sintering at the temperature of heating within an economically practical time interval of heating. As the major part of the sintering is obtained by reason of the iron or nickel content of the metal powder, functioning as the bonding agent for the refractory metal powder, the amount of pressure employed varies somewhat with respect to variations in the ratio between the diamonds and metal powder. The lower the diamond content the lower the pressure required.

In the practice of the present invention the practice of a two-stage sintering operation, the first at atmospheric pressure under conditions inhibiting surface oxidation of the metal particles and the second under reduced pressures up to a high vacuum, each at approximately the same temperature approximating but below the decomposition temperature of the diamonds, seems productive of the best results.

The first heating is accomplished while the metal powder-diamond mixture is in the mold and is preferably conducted in an electric resistance mufile furnace under a positive pressure of a non-carburizing reducing gas such as hydrogen or carbon monoxide. The time interval of sintering under these conditions is limited to one neces sary to impart to the mixture a strength and rigidity enabling the article to be removed from the mold without fracture and to be disposed in position within the vacuum furnace.

The particular type of vacuum furnace employed in the second heating may be varied widely without essential departure from the present invention. This heating is one designed to effect degasification of the sintered product during sintering. Tungsten and molybdenum metal powders, particularly those obtained by the hydrogen reduction of the metal oxide, contain large quantities of adsorbed or occluded hydrogen. This gas is particularly undesirable as a constituent of the sintered product particularly where the low melting alloy filler consists in part of silver, and particularly where the surfacing metal on the diamond consists of silver.

Heating of the metal powder-diamond mixture in vacuum is best accomplished by means of high frequency currents. Several different apparatus are available in the art for this purpose, and such apparatus, per se, forms no part of the present invention.

As one specific embodiment of the present invention, but not as a limitation thereof, the prac tice followed in the production of a lens grinding tool will be described. A lens grinding tool consists of an annular body consisting of an abrasive composition including small particle sized diamonds as the abrasive constituent which is attached concentrically onto the end of a revoluble rod. the forward end of the body being beveled off between the inner and outer edges to provide a plane surfaced grinding face lying at an angle to the rod axis. The rod is usuallymounted to be rotatable axially and to be movable axially and vertically into pressure contact with the surface of a lens blank fixedly sustained horizontally.

The inside and outside diameters of the annular abrasive body varies widely depending upon the size of lens to be ground. For example, an annular body having an inside diameter of 1V2 inches, an outside diameter of 2 inches and a height of about /2 inch is usually employed in the grinding of lenses of from 2 to 3 inches.

This tool in accordance with the Present invention is formed in the following manner:

Diamond fragments passing mesh but not passing mesh, surfaced with silver metal to a depth of at least 1X10 millimeters by electronic projection in accordance with the invention of my Patent No. 2,103,623, above identified, or alternatively surfaced by any of the other methods with a coating of silver of at least 1 to 2 microns thickness, are dispersed uniformly throughout a mixture of tungsten and iron metal powders containing 10% Fe and 90% W, the Fe powder having a particle size passing 400 mesh and the W powder having a particle size passing 200 mesh. One part diamonds to from 5 to 15 parts (by weight) of the Fe.W mixture, preferably 10 parts, is employed.

The mixture is placed in a refractory metal mold, surfaced interiorly with an inert refractory metal oxide, preferably A1203, compacted therein under a pressure of from 1500 to 5000 pounds per square inch, preferably about 2500 to 3000 pounds per square inch following which the mold and compacted mixture therein is placed in an electrically heated muffle furnace with a tight fitting cover thereon and heated for a time interval of about half an hour to a temperature approximating 800 C.

Following this heating, the semi-sintered product is removed from the mold and is placed in d vacuum furnace designed to effect a heating of the product by induced high frequency electric currents, in which furnace the product is gradually heated to a temperature within the range 700 to 800 C. under a continuously maintained high vacuum, the heating bein continued until no further evolution of gases are obtained from the sinteredproduct.

Following thorough degasification of the sintered product in this manner, the product is permitted to cool to atmospheric temperatures in the high vacuum and after breaking the vacuum in the furnace, the sintered product is removed as rapidly as possible and is immersed in a molten bath of a silver soldering alloy having a melting point not higher than about 800 C.

Silver soldering alloys, per se, are old and well known in the art, and per se, form no part of the present invention, except in the combination described. These alloys are ternary alloys of silver, copper and zinc in varying proportions having melting points within the range 650 C. to 870 C. The alloys containing 45-80% Ag, 30-l6% Cu, and 25-4% Zn are the most suitable for the purposes of the present invention.

Upon immersion of the sintered product in the molten silver solder, the porous structure of the product soaks up the molten silver solder which fuse secures the silver-surfaced diamonds in their dispersed position within the porous framework of the sintered tungsten and iron powder-forming, on cooling, a solid metallic mass which has considerable strength and rigidity but which may be weld secured in position upon the end of a rod for use as a lens grinding tool.

In placeof tungsten metal powder I may employ molybdenum metal powder or mixtures of the two. In place of iron metal powder I may employ nickel metal powder or mixtures of the two. I

It is believed apparent from the above disclosure of the present invention that the same may be widely modified without essential de parture therefrom and all such modifications and the scope of the following claims.

What I claim is:

1,, The method of forming an abrasive composition of matter consisting of small particle sized diamonds dispersed throughout a metallic matrix which comprises surfacing the diamonds with a metal selected from the group of metals consisting of Ag, Au and Cu, forming a mixture of metal powders consisting of a refractory metal and a bonding metal therefor, the amount of the bonding metal being at least sufficient to' effect a sintering of the refractory metal at temperatures approximating but not over 800-850" C., incorporating the metal surfaced diamonds in said metal powder mixture in an amount within the range one part diamonds to from to 15 parts of the metal powder, by weight, compacting the diamond-metal powder mixture, heating the compacted mixture under non-oxidizing conditions to a temperature approximating 800-850 C. at atmospherie pressure for a short time interval to sinter the same, heating the sintered product under reduced pressures for an extended time interval to a temperature within the range 700-800 C. to degasify the san'ie, immersing the degasifled product in a molten silver solder alloy having a melting point not exceeding 800 C., and removing and cooling the resultant silver-solder impregnated sintered product.

2. The method of forming an abrasive composition of matter consisting of small sized diamonds dispersed throughout a metallic matrix which comprises surfacing the diamonds with a non-carbide forming protective metal of the group consisting of Ag, Au and Cu, embedding the surfaced. diamonds in a mass of metal powder consisting of tungsten and a bonding metal selected from the group of metals Fe and Ni, the amount of said bonding metal relative to the refr'actory metal being at least sufficient to provide a sintering temperature of the said mass of 800 to 850 C., compacting the said mass with the embedded diamonds therein, heating the comadaptations are contemplated as may fall within pacted mass to BOO-850 C. in a non-carburizing reducing atmosphere for a time interval providing a porous sintered structure in said mass, heating the sintered product under reduced pressures for an extended time interval to atemperature within the range ZOO-800 C. to degasify the porous sintered structure and immersing the degasified product in a molten metal bath consisting of a silver-solder alloy having a melting point not over 800 C. to impregnate the porous metal structure therewith.

3. The method of. forming an abrasive composition of matter consisting of small sized diamonds dispersed throughout a metallic matrix which comprises surfacing the diamond particles with silver, embeddingthe silver-surfaced diamond particles in a mass of metal powders consisting of mixture of tungsten and iron, the iron content of said mixture approximating 10%, compacting the said mass with its contained dia- .monds, heat-treating the compacted product in a. non-carburizing reducing atmosphere to a temperature within the range 800-850 C. for a time interval adapted to sinter the same into a porous metal structure, heating the sintered product under reduced pressures for an extended time interval to a temperature within the range TOO-800 C. to degasify the said porous metal structure and immersing the degasified product in a molten metal bath consisting of a silversolder alloy having a melting point not over about 800 C. to impregnate the poroous metal structure therewith.

4. The method of forming an abrasive composition of matter consisting of small sized diamonds dispersedv throughout a metallic matrix which comprises surfacing each of the said diamonds with a metal selected from the group of metals Ag, Au and Cu, dispersing the metal surfaced diamonds in a mass of metal powder consisting of a mixture of tungsten and iron metal powder containing tungsten and 10% iron with the tungsten powder passing 200 mesh and the iron passing 400 mesh and with 10 parts of the metal powder to each 1 part diamond, by weight, compacting the diamond-metal powder mixture, heating the compacted product under a.

positive pressure of a non-carburizing reducing gas to a temperature of 800-850 C. to sinter the metal powder into a porous metal structure, heating the sintered product under reduced pressures for anextended time interval to a temperature within the range 700-800" C. to degasify the porous metal structure, and immersing the degaslfied porous metal structure in a molten bath of a silver-solder alloy having a melting point below 800 C. to impregnate the porous metal structure with said silver-solder alloy.

HERMANN KO'I'I. 

